One type of fiber optic temperature sensor is based on the decay time of the fluorescence emitted by a phosphor, which serves as the active element of its thermal probe. An optical fiber connects the phosphor element, either directly or via a patch cable, to a controller. The controller provides the optical excitation of the phosphor, and receives and analyzes some of its fluorescent emission. When the decay time of the fluorescence exhibits a monotonic dependence on the temperature of the phosphor, a thermometer can be implemented once that dependence is established.
The phosphor is usually in the form of a mini-rod made from a single-crystal material. When the appropriate material is used, exceedingly high temperatures can be measured. In a conventional design, the cylinder is placed inside a closed-ended sheath and the optical fiber is butt-coupled to it (Fernicola et al., Rev. Sci. Instrum. 71, 2938 (2000)). In a more robust construction, the phosphor mini-rod in the form of a doped single crystal is grown directly onto a corresponding undoped single-crystal lead fiber (U.S. Pat. No. 6,045,259). With the latter approach, a maximum temperature of 1,600° C. could be measured by the thermal probe (Kennedy and Djeu, Sensors and Actuators A100, 187 (2002)). However, the fabrication of the probe in either case is very laborious. Moreover, the maximum concentration of a dopant that can be incorporated into a single crystal is often limited by segregation effects, thus restricting the parameter space that can be explored in the optimization of the phosphor.
Microspheres have been used in fiber optic sensors in the past (U.S. Pat. No. 5,496,997 and German patent DE102009005162A1). U.S. Pat. No. 5,496,997 teaches a sensor incorporating an optical fiber and a solid porous inorganic microsphere. In that invention the microsphere is attached to one end of an optical fiber by means of an adhesive. DE102009005162A1 took it one step further. This later invention teaches the creation of a recess at the end of the fiber with matching radius of curvature to that of the microsphere. Then the two parts are joined with an adhesive. In both approaches the tip consists of a contiguous solid structure. The present invention is yet another version of the microsphere approach.